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DEWATERING CONTROL OF GROUNDWATER - PE Civil Exam

9-1 9. DEWATERING CONTROL OF GROUNDWATER Construction of buildings, powerhouses, dams, locks and many other structures requires excavation below the water table into water-bearing soils. Such excavations require lowering the water table below the slopes and bottom of the excavation to prevent raveling or sloughing of the slope and to ensure dry, firm working conditions for construction operations. GROUNDWATER can be controlled by means of one or more types of DEWATERING systems appropriate to the size and depth of the excavation, geological conditions, and characteristics of the soil. Construction sites are dewatered for the following purposes: 1- To provide suitable working surface of the bottom of the excavation.

The header pipe (15-30 cm diameter, connecting all wellpoints) is connected to a vacuum (Suction assisted self – priming centrifugal or piston) pump. The wellpoints can lower a water level to a maximum of 5.5 m below the centerline of the …

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Transcription of DEWATERING CONTROL OF GROUNDWATER - PE Civil Exam

1 9-1 9. DEWATERING CONTROL OF GROUNDWATER Construction of buildings, powerhouses, dams, locks and many other structures requires excavation below the water table into water-bearing soils. Such excavations require lowering the water table below the slopes and bottom of the excavation to prevent raveling or sloughing of the slope and to ensure dry, firm working conditions for construction operations. GROUNDWATER can be controlled by means of one or more types of DEWATERING systems appropriate to the size and depth of the excavation, geological conditions, and characteristics of the soil. Construction sites are dewatered for the following purposes: 1- To provide suitable working surface of the bottom of the excavation.

2 2- To stabilize the banks of the excavation thus avoiding the hazards of slides and sloughing. 3- To prevent disturbance of the soil at the bottom of excavation caused by boils or piping. Such disturbances may reduce the bearing power of the soil. Lowering the water table can also be utilized to increase the effective weight of the soil and consolidate the soil layers. Reducing lateral loads on sheeting and bracing is another way of use. A number of methods are available for controlling the inflow of water into an excavation; the choice of method will depend on the nature and permeability of the ground, the extent of the area to be dewatered, the depth of the water table below ground level and the amount by which it has to be lowered, the proposed methods of excavation and ground support, the proximity of existing structures, the proximity of water courses etc.

3 9-2 The available methods of GROUNDWATER CONTROL fall into the following basic groups: 1. Surface water CONTROL like ditches, training walls, embankments. Simple methods of diverting surface water, open excavations. Simple pumping equipment. 2. Gravity drainage. Relatively impermeable soils. Open excavations especially on sloping sites. Simple pumping equipment. 3. Sump pumping (see below) 4. Wellpoint systems with suction pumps. (See below) 5. Shallow (bored) wells with pumps. (See below) 6. Deep (bored) wells with pumps. (See below). 7. Eductor system (See below) 8. Drainage galleries. Removal of large quantities of water for dam abutments, cut-offs, landslides etc. Large quantities of water can be drained into gallery (small diameter tunnel) and disposed of by conventional large scale pumps.

4 9. Electro-osmosis. Used in low permeability soils (silts, silty clays, some peats) when no other method is suitable. Direct current electricity is applied from anodes (steel rods) to cathodes (well-points, small diameter filter wells) Exclusion methods; (not covered in this note) 1. Ground freezing (ammonium brine refrigeration or liquid nitrogen refrigeration). All types of saturated soils. 2. Slurry trench cut-off walls with bentonite or native clay and Diaphragm concrete walls. All soils. Curtain walls around excavations with flat buckets. 3. Impervious soil barrier. All soils. Relatively shallow applications (5-6m max.). Back-hoes form the clay filled barriers some distance from the excavation boundaries.

5 4. Sheet piling. All soils except soils with large boulders. 5. Secant (interlocked) piling or tangent piling with grouting in between. All soils except boulders. 6. Compressed air. All types of saturated soils and rock. Applications in tunnels, shafts and caissons. 7. Grouted cut-offs (jet grouting, cementatious grouts, chemical grouts etc.) and sump pumping A sump is merely a hole in the ground from which water is being pumped for the purpose of removing water from the adjoining area (Fig ). They are used with ditches leading to them in large excavations. Up to maximum of 8m below pump installation level; for greater depths a submersible pump is required. Shallow slopes may be required for unsupported excavations in silts and fine sands.

6 Gravels and coarse sands are more suitable. Fines may be easily removed from ground and soils containing large percent of fines are not suitable. If there are existing foundations in the vicinity pumping may cause settlement of these foundations. Subsidence of adjacent ground and sloughing of the lower part of a slope (sloped pits) may occur. The sump should be preferably lined with a filter material which has grain size gradations in compatible with the filter rules. For prolonged pumping the sump should be prepared by first driving sheeting around the sump area for the full depth of the sump and installing a cage inside the sump made of wire mesh with internal strutting or a perforating pipe filling the filter material in the space outside the cage and at the bottom of the cage and withdrawing the sheeting.

7 Two simple sumping details are shown in Figures 2 and 3. systems A wellpoint is cm diameter metal or plastic pipe 60 cm 120 cm long which is perforated and covered with a screen. The lower end of the pipe has a driving head with water holes for jetting (Fig ,b). Wellpoints are connected to cm diameter pipes known as riser pipes and are inserted into the ground by driving or jetting. The upper ends of the riser pipes lead to a header pipe which, in turn, connected to a pump. The ground water is drawn by the pump into the wellpoints through the header pipe and discharged (Fig ). The wellpoints are usually installed with 3m spacing (See Table 1). This type of DEWATERING system is effective in soils constituted primarily of sand fraction or other soil containing seams of such materials.

8 In gravels spacing required may be too close and impracticable. In clays it is also not used because it is too slow. In silts and silt clay mixtures the use of well points are aided by upper ( long) compacted clay seals and sand-filtered boreholes (20cm 60cm diameter). Upper clay seals help to maintain higher suction ( vacuum ) pressures and sand filters increase the amount of discharge. Filtered boreholes are also functional in layered soil profiles (Figures ,b,c,d,e) 9-5 9-6 Table Typical spacings for some common soil types and the approximate time required for effective drawdown Soil Typical Spacing (m) Time (days) Silty sand 7-21(Could be longer) Clean fine to coarse sand 3-10 and sandy gravel Fine to coarse gravel 1-2 The header pipe (15-30 cm diameter, connecting all wellpoints) is connected to a vacuum (Suction assisted self priming centrifugal or piston) pump.

9 The wellpoints can lower a water level to a maximum of m below the centerline of the header pipe. In silty fine sands this limit is 3-4 m. Multiple stage system of wellpoints are used for lowering water level to a greater depth. Two or more tiers (stages) are used. (Fig ). More pumps are needed and due to the berms required the excavation width becomes wider. A single wellpoint handles between 4 and m3/hr depending on soil type. For a 120 m length (40 at 3 m centers) flow is therefore between 160 and 24 m3/hr. Nomograms for selecting preliminary wellpoint spacing in clean uniform sand and gravel, and stratified clean sand and gravel are shown in Figures and 9-7 9-8 Horizontal wellpoints are used mainly for pipeline water.

10 They consist of perforated pipes laid horizontally in a trench and connected to a suitable pump. Wells Shallow wells comprise surface pumps which draw water through suction pipes installed in bored wells drilled by the most appropriate well drilling and or bored piling equipment. The limiting depth to which this method is employed is about 8 m. Because wells are prebored, this method is used when hard or variable soil conditions preclude the use of a wellpoint system. These wells are used in very permeable soils when wellpointing would be expensive and often at inconveniently close centers. The shallow well can be used to extract large quantities of water from a single hole.